A study of data collected from 2001-2004 estimated 35.4% of adults aged 40 and older have balance problems, with odds of dysfunction increasing significantly with age (Agrawal et al., 2009). Humans and other mammals are vulnerable to permanent deficits of hearing and balance that arise when hair cells are killed by loud sounds, drugs, infections, and other causes. Fish, amphibians, reptiles, and birds can regenerate hair cells and recover sensory function after supporting cells divide and give rise to replacement hair cells. The unique belts of filamentous actin which bracket the apical junctions of supporting cells in the balance organs of rodents and humans become substantially reinforced postnatally (Burns et al., 2008). Their counterparts in chickens, however, remain thin throughout life. The time course of growth of the F-actin belts nearly perfectly correlates with a decline in supporting cell spreading and proliferation, and the belts approach their maximal thickness as the total number of hair cells in the utricular epithelium plateaus. Therefore, pharmacological agents and adenoviral vectors expressing proteins which have been shown to reduce or eliminate these belts will be used to determine if the belts are placing a limit on the ability of supporting cells to spread, divide, or differentiate into hair cells. Additional research will investigate potential mechanisms by which reinforced actin belts influence the regenerative capacities and/or normal functions of supporting cells. The dynamics of the actin within the belts will be characterized since polymerization and contraction of the belt's filamentous actin has been shown to regulate cell signaling and tissue remodeling. This will be accomplished by characterizing the expression of actin binding proteins which participate in filament dynamics and by using techniques which directly measure the rate of actin turnover within the belts. The ability of the belts to generate tension will also be assessed with active force measurements. Academic training to aid in the proposed research endeavors will focus on strengthening the background in inner ear biology to complement previous training in biomedical engineering and quantitative analysis. The broad, long- term objectives of the project are to determine if reinforced belts of filamentous actin in mammalian supporting cells play a part in limiting hair cell regeneration. PUBLIC HEALTH RELEVANCE: This research could identify the reasons behind why humans cannot regenerate hair cells, which are vital for sensing sound and gravity. As a result, future therapies could be developed which restore hearing and balance to patients with permanent hair cell deficits.